3. SEARCH AND DISCOVERY

The story of the discovery and characterization of LSB galaxies as important
members of the general galaxy population began in 1963 with the publication
of the David Dunlap Observatory (DDO) Catalog of Galaxies
by Sydney van den Bergh (van den Bergh 1959).
This catalog consists of galaxies which exhibit a diffuse
appearance and that have angular sizes larger than three arc minutes.
While the DDO objects are the first bona fide collection of a sample of
LSB galaxies, they are not at all representative of the phenomenon. The
galaxies contained in the DDO catalog are exclusively of low mass (some
are members of the Local Group). This has fostered an erroneous perception
that all LSB galaxies are dwarf galaxies. Today, we know that all masses of
galaxies have representation in the LSB class.

The first substantial contribution to our understanding of LSB disk
galaxies was made
by William Romanishin and his collaborators Steve and Karen Strom in 1983
(see Romanishin et al.
1983). They derived their sample from the
Uppsala General Catalog of Galaxies (UGC - Nilsson
1973). The UGC is
a diameter selected catalog and is therefore less sensitive to surface
brightness selection effects compared to galaxy selection based on
apparent flux (see also McGaugh
et al. 1995). As a result, the UGC does
contain some LSB disk galaxies but most have µ0
in the range 22-23.0 mag arcsec-2. In early 1984, Allan
Sandage and his collaborators published some of the first results of the
Las Campanas Photographic Survey of the
Virgo Cluster. Contained in
those papers were some dramatic examples of dwarf galaxies in the
Virgo cluster which were quite diffuse. The
existence of such diffuse objects
in a cluster was very interesting as this environment should prove
hostile to their formation and survival. In addition, Ellis, Grayson and Bond (1984)
discovered additional examples of extremely diffuse galaxies
in the field. This immediately raised the possibility that perhaps galaxies
like this were common but, for the most part, not yet detected and cataloged.
If so, perhaps these faint diffuse
galaxies were the source of the enigmatic QSO absorption line systems.
These considerations forged the Impey/Bothun collaboration as we collectively
wondered if the Sandage survey had missed galaxies of even lower surface
brightness. To answer this question we enlisted the help of David Malin in
Australia. Ultimately, we were trying to improve the determination of the
galaxy luminosity function by concentrating on those galaxies which would
be the most difficult to detect, due to extreme diffuseness. We had no idea
if such extreme LSB galaxies really existed; to say we knew what we were doing
would really exploit the advantage of hindsight!

Malin's method of photographic amplification had been used to find low surface
brightness shells and other tidal debris around normal galaxies, and it
could be extended to find entire very LSB galaxies. Indeed,
Malin already had anecdotal evidence that whenever he Malinized a plate,
he would find these ``faint little buggers'' everywhere. So David agreed
to Malinize selected one square degree areas of the
Virgo cluster from
which several small diffuse objects emerged. Skeptical
colleagues insisted that the peculiar collection of faint smudges that could
be seen were all artifacts of the processing, water spots, or specks of dust.
Even the detection of 21-cm emission from one of these ``plate flaws'' did
not assuage one particularly recalcitrant referee. With youth, low pay
and foolishness on our side,
we persisted in our efforts to verify the reality of the smudge galaxies.

In late 1985 and early 1986 we used the Las Campanas 100-inch telescope for
CCD imaging the very diffuse galaxies found in the Malinization process
(all of which turned out to be real). Most of these galaxies were devoid
of structure. However, one had what appeared to be very faint spiral
structure which was connected to a point-like nuclear region. On the
Palomar Sky Survey, this nuclear region is unresolved with no associated
nebulosity apparent. This was one of the few Malin objects bright enough
for optical spectroscopy, and on May 1986 at the Palomar 200-inch telescope,
Jeremy Mould and Bothun took a spectrum of its nucleus. Astonishingly, the
spectrum exhibited emission lines at a redshift of z = 0.083, or
a recessional velocity of about 25,000 kms-1. Now, we had
pursued the Malinization process on UK Schmidt plates of the
Virgo cluster in order to find extremely LSB
galaxies in the cluster. Virgo has a mean recessional velocity of
1150 kms-1, so this nucleated object clearly was far beyond
Virgo.
Since the total angular size of the object on our CCD frame was approximately
2.5 arcminutes, quick scaling then indicated that if a galaxy like this was
indeed in Virgo then its angular size would be a degree. If it
were as close
as the Andromeda Galaxy its angular size would be about 20
degrees and of course we would look right through it without noticing
it. This seemed absurd, and there was a good chance that this strange
galaxy was a composite system, consisting of a background emission line
galaxy shining through a foreground dwarf.

In October 1986, 21-cm observations at Arecibo revealed the characteristic
signature of a rotating disk galaxy whose systemic velocity was equal to
that of the emission line object. The accidental discovery of
Malin 1
(Bothun et al. 1987)
strongly confirmed Disney's original speculation of
the existence of ``crouching giants.'' The existence of
Malin 1 (type
D in the previous example) certainly implied a non negligible space density
of these kinds of objects. The properties of
Malin 1 are described in detail
by Impey and Bothun
(1989). Recent H I observations of
Malin 1 using the VLA
by Pickering et al.
(1997) confirm the presence of a greatly
extended gaseous disk around the normal bulge component of the galaxy.

The remaining ``smudges'' in the Virgo cluster area did not turn out
to be as spectacular as Malin 1. These smudges were most likely
LSB dwarf galaxy members of the Virgo cluster. Their discovery and
characterization by Impey, Bothun
and Malin (1988) readily showed
that the faint end slope of the LF in clusters of galaxies was
significantly steeper than previous measured. In the case of
Virgo,
this meant there were galaxies in the range MB = -12
to -16 which were below the isophotal limits of the plate material used
by Sandage. Most of these have µ0 fainter than
24.5 mag arcsec-2 and l larger
than 1 kpc. The presence of these diffuse galaxies at modest luminosities
increased the faint end power law slope of the LF to a value of -1.55;
significantly steeper than the value of -1.1 which was thought to
hold for clusters and perhaps the field (see
Efthasthiou et al. 1988;
Loveday et al. 1992;
Marzke et al. 1994).

The success of the Malin hunt for very diffuse galaxies prompted three new
surveys. The first relied on the goodwill of Jim Schombert, who was a
Caltech Postdoc associated with the second Palomar Sky Survey. Jim was the
quality control person and thus had direct access to the plates themselves
for a limited period of time before they were secured in the vault. This
allowed an opportunity for hit-and-run visual inspection in pre-defined
declination strips to search for diffuse galaxies with sizes larger than
one arcminute. This produced the catalogs of
Schombert and Bothun (1988)
and Schombert et al.
(1992). A second survey was initiated in the
Fornax cluster using the Malinization technique in
order to compare the results to Virgo. These objects are cataloged and described in
Bothun et al. (1991)
which built on the earlier work of
Caldwell and Bothun (1987). The
third survey was initiated with Mike Irwin at
Cambridge and this involved using the Automatic Plate Machine (APM) to
scan UK Schmidt plates using an algorithm optimized to find galaxies of
low contrast. This forms the most extensive catalog of LSB galaxies to
date (Impey et al. 1996).

The goals of these new surveys was to discover, using various techniques,
the extent and nature of this new population of galaxies which had low
contrast with respect to the sky background and hence have remained
undetected and uncataloged. The importance of discovering this new
population cannot be overstated. The existence of LSB galaxies is a
clear signal
that the samples from which we select galaxies for detailed follow-up studies
are incomplete, inadequate and biased. These surveys are now complete and
most of the results have been published. They have opened up a new field
of inquiry in extragalactic astronomy. Detailed studies of the properties
of individual LSB galaxies, and the class as a whole, has resulted in
a number of recent Ph.D. theses: Knezek (1993),
McGaugh (1992),
Sprayberry (1994),
Dalcanton (1995),
de Jong (1995),
Driver (1995),
de Blok (1997),
O'Neil (1997), and
Pickering (1997).

Figure 2. An example from the Texas survey (e.g.,
O'Neil et al. 1997a) of
an extremely diffuse object. The image size is
1.2 x 1.5 arc minutes
and the galaxy looks to be at least one arc minute in ``diameter'' at
a probable redshift of 4-6,000 km/s. A better presentation of these
diffuse galaxies can be found at